Patent Document 1 below discloses that, in a gerotor pump, contacting surfaces of both of an inner rotor and an outer rotor are given a surface treatment to be coated with a carbide or nitride film to improve abrasion resistance. Also, as a treatment of the contacting surfaces of both of the inner rotor and the outer rotor, a steam treatment has conventionally been known. Meanwhile, Patent Document 2 below discloses a configuration in which an entire surface of one of rotors is coated with a soft material having a self-lubricating property and an entire surface of the other of the rotors is given a surface treatment (plating process) to be coated with a hard material of appropriate thickness.